1. Field of the Invention
The present invention relates to a method of detecting objects that change with time, see FIG. 1, by means of a radar with synthetic aperturexe2x80x94a SAR radar. An important application is detection from an airborne platform of objects concealed in forest vegetation, which change their character, position, orientation or a combination thereof between two or more radar overflights. In such a situation, optical radiation, infrared radiation and microwave radiation are blocked or fluctuate to such an extent that it is not possible to carry out such detection using prior-art technique.
2. Description of the Related Art
SAR is a known technique for two-dimensional high-resolution ground mapping. A platform, such as an aircraft or satellite, moves along a nominal straight path and illuminates a large ground area by means of an antenna. Short pulses, or alternatively long coded signals compressed by means of pulse compression technique, are transmitted from the antenna and the return signal from the ground is received by the antenna and registered by the system along the straight path. By signal processing, high resolution is accomplished both along and transversely of the straight path. A condition for this is that the position of the antenna is known or can be calculated within a fraction of the wavelength and that the radar system is coherent, i.e, the relative amplitude and phase of the transmitted and received radar signal are known. Moreover, the ground and the propagation conditions have to be invariable during the overflight. The optimum geometric resolution that can be provided by means of SAR is determined by centre frequency fc and bandwidth B of the transmitted signal and the projection angle "psgr" between the surface normal of the ground and the image normal {overscore ("psgr")}, and the aperture angle xcex94xcex8, over which the ground area is illuminated by the antenna, along the straight path according to the formula                               Δ          ⁢                      xe2x80x83                    ⁢          A                =                              c            2                                Δ            ⁢                          xe2x80x83                        ⁢                                          f                c                            ·              B              ·              Δϑ              ·              cos                        ⁢                          xe2x80x83                        ⁢            ψ                                              (        1        )            
The SAR technique has been applied in a very wide frequency range, about 20 MHz-100 GHz. The choice of frequency (wavelength) determines largely which ground structures are reproduced since the backscattered return signal is affected above all by structures whose extent is of the wavelength order. Moreover, the wavelength determines the capability of penetrating different media, i.e. the penetration of the wave generally increases with a decreasing frequency. For example, this method can be used to adapt the penetration to reproduce different ground layers. In connection with, for instance, vegetation, the attenuation is insignificant for frequencies below 100 MHz and very great for frequencies above 10 GHz. Thus the capability of penetrating vegetation decreases gradually with an increasing frequency, and a practical limit for detecting objects concealed in vegetation is about 1 GHz. SAR systems which operate below and above 1 GHz, respectively, are in the following thus referred to as low-frequency and high-frequency systems, respectively.
The SAR technique can be accomplished with a small and a great fractional bandwidth (ratio between bandwidth and centre frequency), so-called narrowband and broadband SAR, respectively. In the case of a small fractional bandwidth, the resolution is coarse, in relation to the wavelength and the resolution element generally contains many scattering elements which are superposed with amplitude and phase. From objects in the nature, thus interference is random, which results in so-called speckle noise. The SAR signal from the ground will also be strongly dependent on the direction of observation of the SAR radar since the relative distance changes between the scatterers within the resolution cell will be great in relation to the wavelength, which results in a significantly changed interference pattern. In the case of a great fractional bandwidth, however, the resolution is of the wavelength order and therefore the interference pattern does not change significantly with the direction of observation. Thus the SAR signal will be almost free from speckle noise.
Detection of stationary objects, such as vehicles or houses, requires that they can be discriminated from the background signal, so-called clutter. The latter originates from objects appearing in nature, such as trees and rocks, which are of the wavelength order or larger. The simplest method of detection thus is based on thresholding the SAR image based on the local statistic distribution for the background. The threshold value is set so that the number of false detections (false alarms) is kept at a low and known level. Consequently detection performance is determined mainly by the signal/clutter ratio. The most important system parameters which primarily control detection performance thus are wavelength and geometric resolution. Secondary system parameters are, inter alia, polarisation and angle of incidence.
It is also possible to use characteristic features for discriminating an object from its background, for example, by predefined patterns from an object database. A condition is, however, that the pattern signature is fairly invariable in space and time, from different directions of illumination and in different backgrounds, and that the geometric resolution is significantly finer than the size of the object. According to experience, the geometric resolution must be at least in the order of 0.3 m or better to achieve a robust detection of objects in the size of a vehicle. This corresponds to about 10xc3x9710 resolution cells being contained in the object. A basic problem of pattern recognition of this kind is, however, that the detection is achieved at the cost of significantly reduced detection capacity. Compared with simple thresholding based on 1 resolution cell per object and given a certain system bandwidth, the detection capability is thus reduced by about 100 times in the case of pattern recognition.
Detection of changes by means of narrowband high-frequency SAR is a known technique for detecting objects which change their character between two points of time. SAR images from two or more straight paths are combined. A drawback of this technique is that the speckle pattern in the two images often decorrelates. The speckle pattern will be fully uncorrelated if the straight paths deviate from each other by more than a critical value. The angle over which the speckle pattern is uncorrelated is inversely proportional to the extent of the resolution volume expressed in half wavelengths.
For high-frequency radar backscattering from forest vegetation, the extent of the resolution volume in the vertical direction is tens to hundreds of wavelengths, i.e. much greater than the wavelength, which means that the speckle pattern is quite uncorrelated also for such a small angular difference as about one degree. Besides, it is a considerably more serious condition that the speckle pattern decorrelates owing to small temporal changes in the vegetation. Thus it is sufficient with changes which are but a fraction of the wavelength for the speckle pattern to change, for example by leaves, needles or branches changing their relative positions on the wavelength scale. This so-called temporal decorrelation dominates the change of the speckle pattern of the vegetation in high-frequency SAR. Furthermore its nature is essentially chaotic since it is caused by uncontrollable environmental parameters, such as wind, moisture and temperature. All in all, this means that the speckle pattern causes an increased noise level in connection with the detection of changes, which together with the deteriorated signal/clutter ratio results in insufficient performance for detection of objects under vegetation.
Broadband low-frequency SAR is a technique for reproducing static objects in forest ground. The low frequencies have the property of penetrating vegetation with insignificant attenuation and only causing weak backscattering from the coarse structures of the trees. By means of low-frequency SAR, static objects such as stationary vehicles can be detected even in very thick forest, which has been demonstrated in recent years. However, a problem is that the number of false alarms is large for a reasonable detection probability. The reason is that a large number of naturally occurring objects, e.g. large trees, big rocks, rock ledges, give a signature which resembles the signature one wishes to detect. For objects of the same order as the resolution, such as vehicles, the signature supplies essentially information on the size of the object whereas the shape of the object is ambiguous. For objects that are much larger than the resolution, for example bigger houses, also the shape can be used, which reduces the number of false alarms.
In view of the discussion above, it would be great progress if the SAR technique could be improved so that detection of changes can be combined with low-frequency SAR with high sensitivity to detection of objects, e.g. concealed in forest vegetation, and at the same time little probability of false alarms. An object of the invention is to solve this problem, through a method of detecting objects that change with time within a ground area, by means of a radar with synthetic aperture, a SAR radar, supported by a platform in essentially rectilinear motion during a synthetic aperture. The method includes the steps of transmitting radar pulses with a fractional bandwidth which is greater than or equal to 0.1 and using in the calculations an aperture angle which is greater than or equal to 0.1 radians, and registering the reflected radar pulses with amplitude and phase. For each pulse, the position of the antenna which transmits the pulse and the antenna which receives the pulse are measured, or calculated, and stored. The method proceeds by generating a two-dimensional SAR image with cylinder geometry from each synthetic aperture, and reproducing the ground area at least twice in succession from synthetic apertures. Starting from the two-dimensional SAR images, the images are matched with each other by a method in which each image position in one image is associated with the same ground area in the other image. Knowing location data of the antennae and based on the fact that the cylinder geometry of the SAR images is projected onto the ground surface, the images are filtered so that only common spectral components of the ground reflectivity are extracted and used in the matching.
The method according to the present invention may further include the steps of backprojecting the SAR images to a three-dimensional calculation grid, where the horizontal separation between the grid points is determined by the ground-projected resolution and the vertical separation between the grid layers is determined by focusing depth with regard to the geometry of the straight paths relative to each other, along circles defined by the intersection between range cylinders and azimuth planes for the respective straight paths; in other words, for each grid point, a value given by interpolation of the SAR signals to the range and azimuth position of the given point is assigned, and the signals are filtered to extract the common spectral components of the ground defined by the intersection between the transfer function of each SAR image projected onto the ground surface. The best match between the images with regard to the vertical position is then selected.
The present invention may also include the steps of using, in the final filtering, the average value within a small area round each image position, and selecting the best image match by maximizing the cross-correlation for the backprojected and filtered SAR signals. In addition, a changed object may be detected through comparison with a threshold value. The method may include using at leas two separated antennae in the antenna array and using the measured differential range differences to decide from which side of the straight path the object originates, and radar frequencies below 1 GHz may be used.